Documentation: dt: keystone: provide SoC specific compatible flags

Keystone2 devices are used on more platforms than just Texas
Instruments reference evaluation platforms called EVMs. Providing a
generic compatible "ti,keystone" is not sufficient to differentiate
various SoC definitions possible on various platforms for the
following reasons:
a) Userspace applications have no way of knowing which SoC they are
functioning, providing the compatible matches provide a mechanism for
them to enable SoC specific functionality. Such userspace applications
are typically automated test framework or SoC custom hardware
acceleration entitlement from a common file system.
b) Provides an accurate hardware description. This allows
SoC specific logic to be run time handled based on
of_machine_is_compatible("ti,k2hk") or as needed for the dependent
processor instead of needing to use board dependent compatibles that
are needed now.

Hence, provide compatible matches for each SoC in the Keystone family.

Acked-By: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>

Documentation/devicetree/bindings/arm/keystone/keystone.txt

@@ -9,12 +9,26 @@ Required properties:
    the form "ti,keystone-*". Generic devices like gic, arch_timers, ns16550
    type UART should use the specified compatible for those devices.
 
+SoC families:
+
+- Keystone 2 generic SoC:
+   compatible = "ti,keystone"
+
+SoCs:
+
+- Keystone 2 Hawking/Kepler
+   compatible = ti,k2hk", "ti,keystone"
+- Keystone 2 Lamarr
+   compatible = ti,k2l", "ti,keystone"
+- Keystone 2 Edison
+   compatible = ti,k2e", "ti,keystone"
+
 Boards:
 -  Keystone 2 Hawking/Kepler EVM
-   compatible = "ti,k2hk-evm","ti,keystone"
+   compatible = "ti,k2hk-evm", "ti,k2hk", "ti,keystone"
 
 -  Keystone 2 Lamarr EVM
-   compatible = "ti,k2l-evm","ti,keystone"
+   compatible = "ti,k2l-evm", "ti, k2l", "ti,keystone"
 
 -  Keystone 2 Edison EVM
-   compatible = "ti,k2e-evm","ti,keystone"
+   compatible = "ti,k2e-evm", "ti,k2e", "ti,keystone"